1. Field of the Invention
This invention relates to a frequency multiplier comprising a phase shifter arranged on a pre-stage of a frequency converter.
2. Discussion of the Background
Recently, mobile communications devices such as cellular phones are developed. These devices are directed to be compact and miniaturized so as to be able to be carried personally or mounted on automobiles and the like. For this purpose, it is required to use for parts of such devices monolithically integrated parts that are compact and lightweight in comparison with hybridization parts. ICs used in such devices are required to operate at a low voltage and a low power consumption rate because cellular phones are mostly driven by batteries.
The radio unit of a mobile communications device has a functional feature of converting the frequency of a base band signal to be conveyed to the receiver to that of an RF signal to be transmitted from or received by an antenna. Generally, the frequency conversion between a base band signal and an RF signal is performed by way of an intermediate frequency stage. The superheterodyne scheme is a typical technique used for the frequency conversion. The superheterodyne scheme involves the use of a local signal having a frequency different from that of the RF signal to effectively eliminate interference between the RF signal and the local signal and provide excellent operating characteristics. The scheme has an additional advantage of providing a favorable spurious characteristic if a filter is used for the intermediate frequency stage and hence is popularly used in commercially available radio sets.
On the other hand, a direct conversion scheme of performing the frequency conversion between a base band signal and an RF signal without using an intermediate frequency stage is also known. The direct conversion scheme involves more factors contributing to degradation in the performance of the radio unit than the superheterodyne scheme. However, the direct conversion scheme has an advantage that the radio unit requires a smaller number of parts than the radio unit based on the superheterodyne scheme and hence is less costly. Research and development efforts have been paid to exploit this advantage on the part of the direct conversion scheme and devices based on this scheme are becoming commercially available.
The interference of the RF signal with the local signal has been an obstacle for the commercialization of devices based on the direct conversion scheme. Such interference occurs because the RF signal and the local signal use the same frequency. A technique has been proposed to avoid the obstacle by reducing the oscillation frequency of the local signal to a half of the frequency of the RF signal and performing a frequency conversion between the base band signal and the RF signal, using a signal having a frequency obtained by doubling that of the local signal by means of a frequency multiplier.
When carrying out a frequency conversion by means of this technique, the frequency (fc/2) of the local signal is doubled. This frequency multiplication can be performed without problem to produce a signal having the doubled frequency when the signal is of a sine wave. However, if the signal is of a rectangular wave, when two signals to be multiplied are in phase or in opposite phase to each other, the multiplication result includes only a direct current component. As a result, no properly multiplied signal is obtained. In this case, 90.degree. phase-shifter is required for obtaining high output with frequency of fc. However, the conventional voltage-driven 90.degree. phase-shifter with a RC/CR circuit has a large phase-error when a rectangular signal is input to the phase-shifter. Conventionally, the waveform of the current input to the phase shifter is no longer sinusoidal wave and consequently the accuracy of phase shift is degraded.